Dissociation-deposition unit for the production of chromium



Oct. 11, 1960 l. E. CAMPBELL EI'AL 2,955,566

DISSOCIATION-DEPOSITION UNIT FOR THE PRODUCTION OF CHROMIUM Filed April 16, 1957 Coolant 1,111,111. :IIIIIIIIA Fig. 4.

INVENTORS Ivor E. Campbell John M. Blocher, Jr. A%RNEY 2,955,566 Patented Oct. 11, 1960 DISSOCIATION-DEPOSITION UNIT FOR THE PRODUCTION F CHROMIUM Ivor E. Campbell, New Albany, and John M. Blocher, Jr., Columbus, Ohio, assignors, by mesne assignments, to hilean Nitrate Sales Corporation, New York, N.Y., a corporation of New York Filed Apr. 16, 1957, Ser. No. 653,246

5 Claims. (Cl. l18-48) This invention relates in general to apparatus for the production and recovery of chromium metal in a highly pure state. More particularly, the invention contemplates the provision of improved apparatus for obtaining chromium metal in pure form from chromium iodides by thermal dissociation of the iodides in vapor form and deposition of the metal on a heated dissociation-deposition surface.

It is now well established that certain metals including, for example, titanium, chromium and zirconium, can be recovered in pure form by contacting their corresponding iodides in the vapor phase with a heated dissociationdeposition filament or surface maintained at a temperature above the dissociation temperature of the metal iodide; such procedure resulting in dissociation of the metal iodide at the dissociation-deposition surface and deposition of the pure metal on that surface. Assuming that an atmosphere of the metal iodide vapor is maintained in contact with the heated dissociation-deposition surface, either by feeding from an external source or by formation in situ from crude metal and iodine liberated during the dissociation reaction, the deposition of pure metal can be effected on a continuous basis and a substantial body of metal will be built up progressively on the original surface.

Heretofore, the foregoing classical van Arkel-deBoer principles have been applied to produce a variety of metals by decomposition of their iodides at directlyheated filament wires or rods which are preferably of the same composition as the depositing metal. Deposition on resistively heated filaments of this type requires close control and regulation of the power input to compensate for variations in the resistive load caused by the gradual build-up of deposited metal on the resistance filaments. Furthermore, upon application of this hot-wire technique in efforts to dissociate chromium iodides for the production and deposition of high-purity chromium on wire dissociation filaments formed of various materials, including chromium, it has been found that the reaction proceeds extremely slowly, because of the limited deposition surface available, and the chromium deposits thus produced tend to consist of coarsely-crystalline, brittle, spiny structures. Furthermore, because of the highly non-uniform nature of these deposits and the consequent large variation in their electrical resistance from point to point, it is difficult to maintain uniform and constant filament temperatures, with the result that filament fail- 'ires are difficult to avoid.

It has been found heretofore that coherent, compact (ieposits of high-purity metallic chromium can be produced in commercial yields by decomposition of chromium iodides and deposition of elemental chromium on indirectly-heated dissociation-deposition elements formed, for example, of refractory substances such as quartz, high-silica glass, silica, and the like (see copending US. applications Serial Nos. 579,955 and 579,970 filed of even date on April 23, 1956). In employing deposition surfaces of this general type, as distinguished from directly-heated fine wire filaments of the depositing metal, it is, of course, necessary to separate the desired pure metal from the refractory surface upon which it was deposited, and, preferably, in such manner as to permit reuse of the deposition element. For the latter reason, the relatively fragile refractory elements of the aboveenumerated types are not altogether satisfactory for largescale operations. On the other hand, it has been considered heretofore that the original deposition surface must be formed of a substantially inert material in order to prevent contamination of the deposited metal.

It is the principal object of the present invention to provide an improved dissociation-deposition apparatus of the general class described incorporating, among other features, a plurality of separate deposition elements, preferably of a reclaimable and reusable type, which are formed of materials such as steel, high-chromium stainless steels containing no nickel, and other nickel-free materials, and which function independently of the heating units associated therewith, thereby eliminating the necessity for frequent adjustment of the power input to the respective heating units.

It is a further object of the invention to provide a dissociation-deposition apparatus which includes a unique mechanism for selectively outgassing the reaction chamber at suitable intervals during a deposition cycle.

It is believed that the foregoing features and objects as well as the invention itself may be best understood by reference to the following description of a specific embodiment thereof taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a broken vertical sectional view of an apparatus constructed in accordance with the principles of the invention;

Fig. 2 is a fragmentary transverse sectional view taken along line 22 of Fig. 1;

Fig. 3 is a transverse sectional detail view, on an enlarged scale, taken along line 3-3 of Fig. 1; and

Fig. 4 is a fragmentary elevational view of a modified form of crude charge retainer for use in the apparatus of Fig. 1.

With reference to Fig. 1 the dissociation-deposition apparatus of the invention comprises a container or reaction vessel 1 of generally cylindrical form, closed at one end 2 and open at its opposite end 3. Secured to or formed integrally with the peripheral edge of open end 3, there is provided an outwardly-directed flange 4. Mounted within the open end 3 of reaction vessel l is a filament carrier assembly, represented in general by reference numeral 5, and consisting of an annular flange 6 adapted to provide a sealed closure in cooperation with flange 4, a tubular chamber 7 in the form of an inverted truncated cone of very slight taper (for clearance), and a filament mounting plate 8. The seal between flanges 4 and 6 is perfected by means of annular gaskets 9 mounted therebetween, with provision being made for evacuating the space between the gaskets through bore 10 formed in flange 4. Suitable cooling coils are also provided to maintain the head gaskets at reasonable operating temperatures.

Mounted on plate 8 are a plurality of individual rcentrant-type dissociation-deposition elements 11, each being of elongated structure, and arranged on plate 8 in two annular, concentric rows or series, as best seen by reference to Fig. 2, with elements 11 extending in parallel spaced relationship within chamber 1. Each of elements 11 comprises a substantially tubular deposition sheath 12 open at one end adjacent plate 8 and being removably disposed about a main tubular core 13 (Fig. 3) which is secured to plate 8 in any suitable manner, as, for example, by welding, as indicated within Fig. 1 by reference numeral 14.

Mounted within each tubular core 13 is a conventional resistance type heating unit 15, such as a coil, suitably supported and insulated (16) from said core. The heating units extend within cores 13 for substantially the entire length thereof and, when energized, are operable to maintain the outer deposition sheath 12 at a dissociation temperature for chromium iodide. I Q

The open end of filament carrier assembiy's is adapted to be sealed for operation by means of a closure cover 17 overlying the upper surface of flange 6 an annular gasket 18 therebetwe'en. The interior -of chamber 5 can be evacuated, via vacuum line 19, to maintain cover 17 in pressure contact with'the unit. I

Extending through cover 17 and mounted suitable fluid-tight ceramic seals 20, are electrical conductor elements 21, 22 and 23. Conductor 21 is connected, as by cables 2'4, to one terminal of each of the resistance heating elements 15. The other terrriinal of each resistance element of the outer annular series is connected, as by cables 25, to conductor element 22, whereas a similar connection is provided for each of the terminals on the resistance elements of the inner annular series through cables 26 to conductor element 23. The respective annular series are maintained 'on separate circuits in the manner described because of thediiference in heat loss characteristics for the depositionelernents of each series. The conductor elements 21-23 are each formed asa conduit and a suitable coolant, "such as water, is supplied to the interior thereof to maintain the conductors at a reasonably low temperature. The vacant area within assembly 5 may also be packed with a suitable thermal and electrical insulating material (not shown) to insulate the electrical assembly from the reaction'vessel.

Extending across thein terior of vessel 1, at a point spaced from the lower ends of the dissociation-deposition elements 11, is a screen or perforated plate or sheet 27 formed of any suitable material which is substantially non-reactive with iodine at the operating temperatures involved. The periphery 'orseree' 27 may be frictionally engaged against the inner wallof vessel 1 at the reduceddiameter portion provided at the lower end of the vessel, or it may be permanently secured thereto as by welding or by means'of ring 28 and angle member 29 provided on the inner wall of the vessel in the embodiment illustrated within the drawings. Mounted on screen 27 and extending upwardly therefrom'towards plate 8 of assembly 5, are a plurality of generally cylindrical screens, or perforated members 30, each surrounding and spaced from a'differ'ent one of the dissociation-deposition elements 11. 7

Screens '27 and 30 serve to define a plurality of chambers concentrically surrounding the dissociation-deposition elements 11 and spaced'therefromin such manner as to maintain crude metallic chromium charge material 31, deposited outside of the screens, at a fixed distance from deposition sheaths 12. In initiating operation of the device, a relatively small amount of elemental iodine is introduced into the interior of reaction vessel 1, and, with the unit fully'as'sembled as illustrated in Fig. 1, the vessel is heated to an elevated temperature suchthat the crude chromium charge will react with the iodine to form chromium iodide which is maintained in vapor form at the temperature of the vessel. Preferably, the system is evacuated to remove air prior to initial heating by a suitable vacuum line (35, infra), and continued evacuation of the reaction vessel during the initial heating period serves to remove'occluded or adsorbed gases which may be evolved. Of course, suitable precautionary measures should be exercised to prevent loss of iodine during the evacuation operation such as by (1) keeping it in a suitably refrigerated sidevessel, (2) storingit in a frangible capsule, or (3) adding the necessary iodine inthe form of anhydrous chromous iodide which. has a much lower vapor pressure. Vessel 1 may be heated by simply placing the entire assembly within an oven or furnace, or; by

.4 means of resistance windings provided adjacent the external wall (not shown). With the chromium iodide in the vapor phase, the heaters 15 are then energized to maintain sheaths 12 of the respective dissociation-deposition elements at a dissociation temperature for the chromium iodide, whereby the iodide will be decomposed at the sheaths, depositing the desired pure chromium metal thereon and liberating elemental iodine for reaction with the crude chromium charge in the formation of additional quantities of chromium iodide.

Thus, assuming charge 31 consists of crude metallic chromium, the reaction vessel is charged with a suitable supply of elemental iodine, evacuated as explained above, and heated to a temperature within the range 550-900 C. to effect reaction between the iodine and crude chromium, thereby establishing an atmosphere of chromium iodide vapor in contact with dissociation-deposition sheaths 12. Elements 12 are then maintained at a dissociatlion temperature for chromium iodide, as, for example, within the range 750ll00 C., to efiect dissociation of the iodide vapor and deposition of pure chromium on the surfaces of sheaths 12. The elemental iodine liberated during the dissociation reaction will diffuse through screens 30 to the crude metal charge for reaction in the formation of additional chromium iodide; the process continuing in cyclic fashion until substantial deposits are available for recovery from the respective sheaths 12.

As described in substantial detail With-in the aforementioned ccpending application Serial No. 579,970, it has been found that in the production of iodide chromium, distinct advantages are obtained if the interior of the reaction chamber is left in' communication with an outgassing unit closed, as by a condensed plug of chromium iodide, and periodically opened to eifect outgassing of the chamber. To accomplish this operation in the apparatus of the present invention, reaction vessel 1 is provided with an outgassing port 32, Fig. 1, in which there is mounted one end of a conduit 33 leading to a chamber 34 which is, in turn, connected to a vacuum line 35. Mounted on one wall of chamber 34, and extending into the conduit 33 so as to be disposed concentrically therein, is a tubular electrical resistance heater 36. Thus, the conduit 33 and heater 36 combine to define a restricted annular passageway communicating between vacuum line 35 and the interior of vessel 1. Disposed exteriorly of conduit 33, so as to surround such restricted passageway, is a helical coil 37 of a coolant line.

With valve 38 of the coolant line open, and with the apparatus in operation for the recovery of chromium in the manner explained above, coil 37 will maintain conduit 33 and the restricted outgassing passageway at a low temperature which is effective to condense a plug of chromium iodide in the outgassing passageway, thereby precluding passage of gases outwardly to the vacuum line35. When it is desired to outgas the reaction vessel, valve 38 is closed and heater 36 is energized, as by means of switch 39, to heat the outgassing passageway to a temperature at which the chromium iodide plug is vaporized, thereby reestablishing communication between the interior of container 1 and vacuum line 35.

The charge retaining elements 27' and 30 have been illustrated as being of woven wire mesh and may be formed of any iodine inert material, such as molybdenum; In lieu of wire mesh, however, such elements can be, fabricated from perforated sheet, expanded metal, ol screen, such as that illustrated in Fig. 4 of the drawings. In this connection, it is found that best resultsare ob. tained with charge retainers which afford the maximum possible ratio of open to total area. Accordingly, charge retaining elements 27 and 30 should be formed with them'aximum open or exposed area permissible from a mechanical standpoint.

All structural parts ofthe apparatus which are subjectto attack by iodine, such as reaction vessel 1, assembly 5, plate 17, etc., can be fabricated of high-chromium stainless steel, or, such a high-chromium stainless steel which is chromized before or after assembly to obtain a higher chromium-alloy surface in contact with the iodine and iodide vapors. Thus, the reaction vessel and other reaction-exposed surfaces of the apparatus can be preconditioned, and subsequently reconditioned as required, to chromize the surfaces normally subject to attack by iodine or iodide simply by contacting such surfaces with chromous iodide vapor while maintaining them at a dissociation temperature for the iodide. In this manner, the chromous iodide will be dissociated in contact with the critical surfaces, depositing substantially pure chromium metal thereon forming a chromium plate in..the case. of ordinary ferrous surfaces, or a higher chromium-alloy coating where high chromium stainless is employed as the material of construction. The use of structural materials of the general class described and preconditioning of these materials to enhance their resistance to attack by iodine and iodide, will insure the production of iron-free deposits of chromium metal during subsequent normal operation of the system.

It is also possible to condition the reaction vessel by coating the interior thereof with molybdenum, or chromium, by conventional metallizing techniques. Alternatively, the vessel can be coated with a heavy chromium electroplate or a thin rhenium electroplate, or, a quartz or molybdenum lining can be fitted throughout the vessel. For example, such a molybdenum liner could consist of thin molybdenum sheet joined together with lock seams, and, if a very tight structure is desired, palladium foil could be used in the seams, and the seams brazed.

As will be readily apparent, deposition of the chromium metal on metallic sheaths formed of steel or highchromium stainless steels, renders it possible to effect recovery of the deposited chromium by mechanical means, such as machining, with recovery of the deposition elements for reuse. Alternatively, recovery of the deposited metal from sheaths 12 can be effected by controlling the deposition phenomenon to establish a separatory weakened Zone between an initial layer and subsequent layers of deposited metal in the manner described in copending application Serial No. 653,243 of Neil D. Veigel, filed of even date with this application, now abandoned.

Having thus described the subject matter of our invention, what it is desired to secure by Letters Patent is:

1. Apparatus for use in recovering chromium metal in substantially pure form from chromium iodide by thermal dissociation of the iodide and deposition of chromium on a heated dissociation-deposition surface that comprises, a dissociation vessel, a plurality of dissociationdeposition elements projecting within said dissociation vessel in spaced parallel relationship to each other, a metallic dissociation-deposition sheath removably mounted around each of said dissociation-deposition elements, means for maintaining an atmosphere of chromium iodide in vapor form in contact with said dissociation-deposition sheaths, and means for indirectly heating said dissociation-deposition elements and associated sheaths to effect dissociation of said chromium iodide and deposition of \ohromium metal on said sheaths.

2. In an apparatus for recovering chromium metal in substantially pure form from chromium iodide by thermal dissociation of the iodide and deposition of chrornium metal on a heated dissociation-deposition surface, the combination of a reaction vessel provided with an open end, a removable closure for said open end, means for elfecting a fluid-tight seal between said reaction vessel and closure, a plurality of dissociation-deposition members mounted Within said reaction vessel in spaced parallel relationship to each other and each including a removable metallic deposition surface and means for maintaining the surface at a dissociation temperature for chromium iodide, means for establishing and maintaining an atmosphere of chromium iodide in vapor form in contact with each of said dissociation-deposition surfaces, and means for selectively outgassing said reaction vessel.

3. In an apparatus for recovering chromium metal by thermal dissociation of chromium iodide in vapor form and deposition of chromium on a heated dissociationdeposition surface, the combination of a reaction vessel having an open end and an outgassing port provided therein, closure means for sealably closing said open end, at least one metallic dissociation-deposition member mounted within said reaction vessel, means for indirectly heating said member to maintain the same at a dissociation temperature for chromium iodide, means for maintaining a highly-heated atmosphere of chromium iodide in vapor form in contact with said dissociation deposition member, an outgassing conduit connected to said outgassing port, means disposed exteriorly of said outgassing conduit for cooling the same over a restricted portion thereof, and heating means disposed within said outgassing conduit to heat the interior thereof over the same restricted portion.

4. In an apparatus for obtaining chromium metal in substantially pure form by thermal dissociation of the vapor of chromium iodide and deposition of chromium metal on a heated surface, the combination of a reaction vessel having an open end, a plate removably mounted within said reaction vessel, a plurality of metallic dissociation-deposition members removably mounted on said plate and spaced from each other, means for indirectly heating said members to maintain the same at a dissociation temperature for chromium iodide, means mounted within said reaction vessel for retaining a crude charge of chromium spaced from each of said dissociationdeposition members, whereby an atmosphere of chromium iodide can be established in said reaction vessel in con tact with said dissociation-deposition members by introducing iodine therein and heating the vessel to a vapor temperature of chromium iodide, means for sealably closing said open end, and selectively operable means for outgassing said reaction vessel.

5. In an apparatus for obtaining chromium metal in substantially pure form by thermal dissociation of the vapor of chromium iodide and deposition of chromium metal on a heated surface, the combination of a reaction vessel, a plate removably mounted in said reaction vessel, a plurality of elongated dissociation-deposition devices mounted on said plate and disposed in parallel spaced relationship to each other, vapor-permeable charge retaining means disposed within said reaction vessel and mounted therein to define crude chromium metal charge retainers surrounding and spaced from each of said dissociation-deposition devices, means for indirectly heating said devices to a dissociation temperature for chromium iodide, an outgassing conduit communicating with the interior of said container, means cooperating therewith to define a restricted outgassing passage, and means for selectively cooling and heating such passage.

References Cited in the file of this patent UNITED STATES PATENTS 

1. APPARATUS FOR USE IN RECOVERING CHROMIUM METAL IN SUBSTANTIALLY PURE FORM FROM CHROMUIM IODIDE BY THERMAL DISSOCATION OF THE IODIDE AND DEPOSITION OF CHROMUIM ON A HEATED DISSOCIATION-DEPOSITION SURFACE THAT COMPRISES, A DISSOCATION VESSEL, A PLURALITY OF DISSOCIATIONDEPOSITION ELEMENTS PROJECTING WITHSAID DISSOCATION VESSEL IN SPACED PARALLEL RELATIONSHIP TO EACH OTHER, A METALLIC DISSOCIATION-DEPOSITION SHEAT REMOVABLY MOUNTED AROUND EACH OF SAID DISSOCATION-DEPOSITION ELEMENTS, MEANS FOR MAINTAINING AN ATMOSPHERE OF CHROMUIM IODIDE IN VAPOR FORM IN CONTACT WITH SAID DISSOCATION-DEPOSITION SHEATS, AND MEANS FOR INDIRECTLY HEATING SAID DISSOCIATION-DESPOSITION ELEMENTS AND ASSOCIATED SHEATS TO EFFECT DISSOCATION OF SAID CHROMUIM IODIDE AND DEPOSITION OF CHROMUIM METAL ON SAID SHEATS. 